Do not use your cell phone in theatre. This is such an important etiquette that it has become somewhat of a law in movie theatres. While people may be willing to forgive you for using the light from your cell phone screen to illuminate the way to your seat, there is absolutely no forgiveness for talking on the phone during the film. Also make sure that you put your phone on silent so that people do not get disturbed or distract by your ringtone.
Do not take pictures of the movie. The flash from the camera is pretty distracting and consequently bound to annoy other people in the theatre. It is also really difficult to understand why a person would want to take a picture of the film. Theatre is a place to watch and enjoy movies, not to take pictures.
Do not rest your knees or feet on the chair or armrest in front of you. You do not want to make the person sitting in front of you uncomfortable and angry by kicking the back of his seat. The ushers will surely tell you to mind your feet or knees if they notice you resting them on the chair or armrest in front. However, things can become really tense if the person sitting in the front chair decides to deal with the matter himself.
Do not leave your eat and move across the aisle too frequently. When nature calls, there is not much you can do about it. However, do not come back only to get up again to get drinks or snacks. Plan out your breaks in such a way that you get everything done in the same trip.
If you do not like something that is happening on the screen, there is no need to shout it out loud. People did not come to the theatre to hear what your thoughts. They are interested in watching the movie, so let them watch it peacefully.
Do not start making out in the theatre, as it will make people around you feel awkward, especially the parents who have brought their kids along. You probably do not want people to be staring at your make out session either.
Do not make too much noise while enjoying your snacks. Try to open your snacks during a particularly loud scene so that the sound of you opening the packet does not disturb others. Also try not to make a lot of noise while crunching your snacks or chewing a gum.